For example, an electric actuator disclosed in JP 11-303966 A is known as an electric actuator using the ball-screw spline of this type. The ball-screw spline includes a cylindrical cylinder, a spline nut arranged at one end of the cylinder, a hollow spline shaft arranged in a hollow portion of the cylinder and guided by the spline nut, and a screw shaft arranged in a hollow portion of the spline shaft.
A screw groove in which balls roll is helically formed in an outer peripheral surface of the screw shaft, and the screw shaft is rotationally driven by a motor fixed to another end of the cylinder. Meanwhile, one end of the spline shaft serves as a ball-nut portion, and a screw groove facing the above-mentioned screw groove of the screw shaft is formed in an inner peripheral surface of the ball-nut portion. The spline shaft is threadedly engaged with the screw shaft through intermediation of a large number of balls.
Further, a plurality of rows of ball rolling grooves are formed in an outer peripheral surface of the spline shaft along an axial direction. The spline nut is assembled to the spline shaft through intermediation of a large number of balls which roll in the ball rolling grooves.
In order to form an infinite circulation path for the balls with respect to the ball-nut portion of the spline shaft, a cutout flat-surface portion is formed on an outer peripheral surface of the one end of the spline shaft. To the cutout flat-surface portion, there is fixed a return tube for returning the balls rolling up to one end of the screw groove of the ball-nut portion to another end of the screw groove. The return tube projects from the outer peripheral surface of the spline shaft to a radially outer side, and hence the ball rolling grooves formed in the outer peripheral surface of the spline shaft are provided within a range in which the ball rolling grooves do not overlap the ball-nut portion.
In the electric actuator structured as described above, when the screw shaft is rotated by the motor, the spline nut fixed to the cylinder functions as a rotation stopper for the spline shaft. Thus, the spline shaft threadedly engaged with the screw shaft moves in the axial direction in accordance with rotation of the screw shaft. That is, the spline shaft is moved by a stroke amount corresponding to a rotational amount of the screw shaft.